Basic Linux commands

The basic components of Linux are:

• Kernel: It is the core component of the Operating System that manages operations and hardware.
• Shell: Shell is a Linux interpreter that is used to execute commands.
• GUI: GUI stands for Graphical User Interface which is another way for a user to interact with the system. But unlike CLI, GUI consists of Images, Buttons, TextBoxes for interaction.
• System Utilities: These are the software functions that allow the user to manage the computer.
• Application Programs: Software programs or set of functions designed to accomplish a specific task.

The most common Shells used in Linux are

• bash: Bourne Again Shell is the default for most of the Linux distributions
• ksh: Korn Shell is a high-level programming language shell
• csh: C Shell follows C like syntax and provides spelling correction and Job Control
• zsh: Z Shell provides some unique features such as filename generation, startup files, login/logout watching, closing comments etc.
• fish: Friendly Interactive Shell provides some special features like web-based configuration, auto-suggestions, fully scriptable with clean scripts

The basic and most commonly used linux commands

1. ls — and its options
   A) ls -list all non-hidden files
   B) ls -lrt — List all (ls) the files in the current directory in long format(l), sorted by modification time(t), oldest first(r- recursive).
   C) ls -lart — List all(ls) the files in the current directory including .(a)in long format(l), sorted by modification time(t), oldest first(r- recursive).
2. hostname- used to obtain the DNS(Domain Name System) name and set the system’s hostname or NIS(Network Information System) domain name.
   A) hostname — display the hostname.
   B) sudo hostname <hostname> — set the hostname.
3. cp — used to copy the file and folder form one directory to another in a system
   -cp [options] source dest
   A) cp -n — no file overwrite.
   B) cp -R — recursive copy (including hidden files).
   C) cp -u — update — copy when the source is newer than dest.
   D) cp *.java test — copy all java files in test directory.
4. cd — used to change the current directory
   A) cd [directory] — go to another mentioned directory.
   B) cd ~ — change to home directory.
   C) cd .. — change to parent directory
   D) cd / — change to root directory
5. mv — Move/ Rename files and directories.
   A) mv -f — force move by overwriting destination file without prompt.
   B) mv -I — interactive prompt before overwrite.
   C) mv -u — update — move when source is newer than destination.
   D) mv -v — destination.
6. ssh — is a network protocol that enables secure remote connections between two systems. We can login to the remote system using ssh.
   A) ssh 192.168.56.101 — ssh without username.
   B) ssh username@hostname_or_ip — ssh with username.
   C) ssh test.server.com -p 3322 — ssh with port number (-p 3322).
7. scp — used to securely copy files over the SSH protocol using the SCP tool
   A) scp fileName user@remotehost:/home/username/destination — copy in the remote system at the destination directly.
   B) scp *.java user@remotehost:/home/username/destination — copy all java files in the remote system at the destination directly.
8. du — display disk space usage
   A) du -h — print sizes in a human-readable format.
   B) du -s — summary of the file system using -s option.
   C) du -sh — print summary of the files in the current directory in a human-readable format.
9. chmod — used to modify the permissions of the file specified by file name to the permissions specified by permissions.
   A) chmod u=rwx,g=rx,o=r myfile.
   a) u — the file owner can read, write and execute the file.
   b) g- the file’s group can read and execute the file.
   c) o — the all other users can only read the file.
   B) chmod 754 myfile (4 stands for “read”, 2 stands for “write”, 1 stands for “execute”, and 0 stands for “no permission.”)
   a) 7 (4+2+1)- the file owner can read, write and execute the file.
   b) 5 (4+1)- the file’s group can read and execute the file.
   c)4 — all other users can only read the file.
10. pwd — used to locate the current working directory.
11. kill — used to terminate processes manually.
    A) kill -9 [pid] -kills the process running at [pid]
12. lsof — list of opened files
    A) lost -U -list of opened files per user.
    B) lsof -i — Files opened by network connections.
    C) lsof -I:[port_num] -Files opened by mentioned port.
13. jps — used to list all the processes that are running on Java virtual machine.
14. cat — the cat command is used for many purposes for text file/s like display, read, file concatenation, modifying, etc.
    A) cat filename — displays the content of filename.
    B) cat filename1 filename2 — displays the content of filename1 and filename2.
    C) cat filename.txt — displays the content of filename with the printed line number for each line.
15. vi — is the text editor used in line for file modifications.
    vi <filename_NEW> or <filename_EXISTING>
    Below are some vi commands used for editing the file.

• i — Insert at cursor (goes into insert mode)
• a — Write after the cursor (goes into insert mode)
• A — Write at the end of the line (goes into insert mode)
• ESC — Terminate insert mode
• u — Undo last change
• U — Undo all changes to the entire line
• o — Open a new line (goes into insert mode)
• dd — Delete line
• 3dd — Delete 3 lines.
• D — Delete contents of line after the cursor
• C — Delete contents of a line after the cursor and insert new text. Press ESC key to end insertion.
• dw — Delete word
• 4dw — Delete 4 words
• cw — Change word
• x — Delete character at the cursor
• r — Replace character
• R — Overwrite characters from cursor onward
• s — Substitute one character under cursor continue to insert
• S — Substitute entire line and begin to insert at the beginning of the line
• ~ — Change case of individual character

There are many more commands which we need to deal with day to day work, but I mentioned a few of them for basic knowledge.